Exposure to nighttime light doesn’t just disrupt your sleep cycle. Even dim light may make cancerous breast tumors develop faster and ultimately grow resistant to tamoxifen, a cancer-fighting drug, according to a new report published in the journal Cancer Research and reported by Medical News Today.

For the study, researchers at Tulane University in New Orleans observed a number of rats with human breast tumors. For the study period, rodent test subjects were put into either normal light and dark conditions (12 hours of light, then 12 hours of complete darkness), or normal light and dim lighting that replaced the period of total darkness at night.

Turns out that the tumor growth rate of rats living under the alternative lighting conditions was 2.6 times faster than that for the normal group. What’s more, researchers found the tumors of the rats living in dim light conditions became completely resistant to tamoxifen. Scientists said the reason this happens is because light—no matter how weak—can suppress the body’s production of melatonin, a hormone that makes tumors able to withstand this cancer treatment.

Researchers noted that their study did not reveal exactly how much light was needed to dangerously disrupt the rats’ melatonin levels, “but we think that it could be as little as the light that comes in the bedroom window from a street light,” said lead study author Steven M. Hill, MD, PhD.

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